Bestatin is known to have the immunopotentiating properties (see U.S. Pat. No. 4,029,547 and U.S. Pat. No. 4,189,604). Bestatin is (2S, 3R)-3-amino-2-hydroxy-4-phenylbutanoyl-(S)-leucine and has the following structure: ##STR1##
We earlier have chemically synthetized some compounds which are related to bestatin in their chemical structure and which are represented by the following general formula: ##STR2## wherein R.sup.1 is ##STR3## where R.sup.3 is hydrogen, chloro, methyl, nitro, hydroxy or amino and n is 0 or 1, and R.sup.2 is a (lower)alkyl having 1 to 6 carbon atoms, hydroxy(lower)-alkyl, alkylthioalkyl, carboxamido(lower)alkyl or carboxy(lower)-alkyl, provided that when R.sup.1 is benzyl and R.sup.2 is isobutyl, the configuration of the compound is (2S,3R,2'R), (2S,3S,2'S) or (2S,3S,2'R) (see U.S. Pat. No. 4,189,604 issued on Feb. 19, 1980). The compounds of the formula (II) are the dipeptides which are physiologically active and inhibit aminopeptidase B, leucine aminopeptidase and Bleomycin hydrolase and enhance the anti-tumor effect of bleomycin.
We have further researched bestatin and its related known compounds, and now we have succeeded to provide further new compounds which are related to bestatin in their chemical structure and which have the aminopeptidase-inhibiting activity and the immunopotentiating activity.
An object of this invention is to provide new compounds which exhibit the immunopotentiating activities. Another object of this invention is to provide processes for the production of these new compounds. Further object is to provide an immunopotentiator comprising these new compounds or their related known compounds. Other objects and utilities of this invention will be clear from the following descriptions.